change color of defroster vent-

I need to change defroster vent color from black to blue on a '69
Coronet. Looking for a procedure that has been tested, this is an
a/c car and currently has the windshield out, so no do-overs.Thanks.

Never had a problem with paint sticking to them as long as they were clean. A few light coats is better than "bombing" them. To go to a lighter color I'd advise misting them with a light or medium colored primer before painting.